The intention of this study is to determine which protein antigens are common or unique to strains of Mycoplasma hominis with the ultimate purpose of defining those antigens important to the human response as well as the antigens prominently displayed on the cell surface. This includes (1) comparison of the antigenic composition of prototypic and wild strains, (2) determining the specificity of these antigens for M. hominis only, (3) determining the location of the antigens on or in the cells and their membranes, and (4) identification of the surface antigens in order to determine whether the apparent antigenic diversity of strains of this species reflects heterogeneity of the surface antigens. Mycoplasma hominis has been implicated as a possible causative agent for numerous diseases of the urogenital tract. The isolation of the organisms and concommitant rise in antibody titer has been shown for patients with symptoms of pelvic inflammatory disease, postpartum fever, and acute pyelonephritis. However, the organisms can also be isolated from apparently healthy persons, and increases in antibody titers can be correlated simply with increasing age. There is also support and lack of support for a causative role for M. hominis with respect to infertility. The data from several serological studies suggests that there is considerable cross-reactivity between strains of the species. It also shares antigenic determinants with other species resident in the human host. The assessment of the role of this organism in disease depends on necessary advances in serodiagnosis and methods of organism identification which require better information concerning antigenic heterogeneity within the species and the antigenic relationship of this species with other mycoplasmas. The antigens to be used will be prepared from a large number of clinical strains. They will be separated by polyacrylamide gel electrophoresis and analyzed after immunoblotting with both hyperimmune rabbit sera and paired human sera. Cross-reactivity between strains will be assessed with immunoblots using antibody fractions prepared by affinity methods to be specific for antigen subsets of narrow molecular weight ranges. Surface disposition will be determined by absorption of antisera and subsequent quantitative analysis or by radioimmunoprecipitation. Peptide mapping in gels will also be done in order to determine the extent of sharing of antigenic determinants.